The present invention relates to a current drive circuit for a magnetic coil, and particularly to a circuit suitable for a magnetic memory.
The present invention can be applied to a circuit for driving a magnetic recording head, a step motor, an electric switch, a relay, and especially to a circuit for driving a magnetic recording head in a magnetic memory.
A magnetic head in a magnetic memory such as a magnetic disc memory or a magnetic tape memory has an induction coil wound around a magnetic core having a gap therein which is comprised of non-magnetic material. A magnetic head drive circuit drives the head and records digital signals in a magnetic recording medium. The circuit has means for applying a predetermined bias voltage to a center tap of the coil, a write circuit for switching the direction of the current flow through the coil in response to an input signal and a read circuit connected in parallel to the coil for reading signals stored in the recording medium. A magnetic memory has a plurality of magnetic heads and a plurality of write circuits and read circuits respectively connected to one of the magnetic heads. These write and read circuits are constructed by using integrated circuit technology. Each read circuit has a differential amplifier, which comprises a pair of transistors having directly connected emitters and a constant current source commonly connected to the emitters.
When the direction of the current flow through a coil is changed in order to record signals, a flyback voltage or in other words a counter electromotive force is induced in the coil and is applied to the bases of the pair of transistors of a corresponding read circuit. The breakdown voltage between the bases of the two transistors is equal to the sum of the forward voltage drop (V.sub.BE) between the base and emitter of each transistor and the backward breakdown voltage (V.sub.EB0) between the emitter and base of each transistor. When the above-mentioned counter electromotive force goes beyond the sum (V.sub.BE +V.sub.EB0), the grounded emitter amplifier constant h.sub.FE of the transistors of the read circuit drops. This results in lowering of the output level of the read circuit. The input resistance R.sub.A of the read circuit, the coil inductance, the coil capacitance and the input capacitance of the read circuit form a resonance circuit. If the amplifier constant h.sub.FE lowers, the input resistance R.sub.A lowers. If the resistance R.sub.A lowers, the damping of the resonant circuit increases and, therefore, a smaller amount of the read out voltage of the head is transferred to the read circuit near the resonance frequency. Thus, the output level of the read circuit drops.